1. Field of the Invention
The present invention relates to a method for producing thin film transistors (hereinafter referred to as TFT), in which is used a metal element such as typically nickel capable of promoting the crystallization of silicon.
2. Description of Related Art
Known are TFT having a silicon film formed on a glass or quartz substrate. Most commercially-available products are amorphous silicon TFT having an amorphous silicon film as the active layer.
TFT are essentially used in active matrix circuits in active matrix-type liquid crystal display devices.
TFT having an amorphous silicon film as the active layer are defective in that they could be put into practical use only for N-channel devices, and that their acting speed is very low. (It may be said that TFT are used only in active matrix circuits because of such defects.)
As one technique for solving the problem, known is a method of using a crystalline silicon film as the silicon film constituting the active layer.
To obtain such a crystalline silicon film, employable is any of laser irradiation or heating.
The former method of laser irradiation comprises applying a laser ray to an amorphous silicon film having been formed through CVD or the like to thereby crystallize the film.
The latter heating method comprises heating an amorphous silicon film having been formed through CVD or the like to thereby crystallize the film.
For the laser irradiation method for crystallization, however, commercially-available laser oscillation devices are not still at practicable level, and therefore the method is problematic essentially in point of the oscillation stability. For that reason, the crystalline silicon films to be obtained in the method are problematic in point of their uniformity and producibility.
On the other hand, the heating method may produce films of stable quality. However, as requiring high heating temperatures, the method is problematic in that glass substrates are difficult to use therein. In addition, since the films formed in the method are in a definite polycrystalline condition, they shall have therein grain boundaries which are unstable factors.
Given that situation, one problem is how to lower the heating temperatures for crystallization. Another problem is of attaining a step of producing films without forming any definite grain boundaries therein.
To solve those problems, the present applicant has developed a technique of crystallizing an amorphous silicon film at a temperature lower than that for conventional crystallization. In the technique developed, used is a metal element such as typically nickel element for the crystallization.
Precisely, the technique is based on a method comprising introducing a predetermined metal element such as typically nickel into an amorphous silicon film followed by heating the film to convert it into a crystalline silicon film.
According to this technique, the crystalline silicon film obtained through heat treatment at about 600.degree. C. or so to which glass substrates are resistant shall have a high degree of crystallinity.
Regarding the quality of the film to be obtained in the technique, the film does not have any definite grain boundaries therein, and is therefore superior to any other conventional, definite polycrystalline silicon films.
However, the method where a metal element is used is problematic in that the metal element used remains in the crystalline silicon film formed.
The metal element remaining in a semiconductor film acts as a trap center and has some negative influences on the characteristics of TFT comprising the film. Therefore, it is desirable to reduce as much as possible the concentration of the metal element remaining in a semiconductor film that constitutes the active layer of TFT.